In a simple SMPS it contains optoisolator at the secondary part and a power switch IC (m02659R) in a way that the feedback is outputted from the optoisolator to the Feedback Pin of the Power switch IC.

If let say there is a faulty optoisolator and there is no feedback to the power switch, what will happen to the DRAIN pin of the power switch and will it blow the secondary circuits?

SMPS IC normally oscillate at a maximum duty cycle (typically about 80% duty) and when feedback > threshold the duty cycle is cut. Check any SMPS IC datasheet for the exact feedback regulation system but that is pretty typical.

So if you lose the feedback the thing will run at full power and the output voltage will rise quite high, and probably blow something.